Components used in high stress, high temperature applications ("high intensity" components) typically are provided with overlay coatings to prevent material oxidation and hot corrosion during service. One type of overlay coating for high intensity components, such as gas turbines, is an overlay coating. An "overlay" coating is conceived and optimized separately from the substrate so that the overlay coating can form an adherent and stable protective oxide (most often Al.sub.2 O.sub.3) at the coating-atmosphere interface. Thermal barrier coatings are a type of overlay coating. A popular overlay coating has a chemical composition of "MCrAlY"--where "M" is nickel, cobalt, or both, Cr is chromium, Al is aluminum, and Y is yttrium.
Certain types of components are subject to particularly high stress and high temperature conditions during use hereinafter called "super high intensity" components). Examples of super high intensity components are jet engine parts and turbo-superchargers. In order to withstand the extreme service conditions, super high intensity components typically are made of a base material known as a "superalloy." Superalloys exhibit high temperature mechanical integrity with an unusual degree of oxidation and creep resistance.
Unfortunately, overlay coatings for high intensity and super high intensity components are not, themselves, immune to material degradation. One cause of material degradation in MCrAlY overlay coatings is the diffusion of constituents from the coating, particularly the diffusion of aluminum.
Some have attempted to improve the performance of MCrAlY coatings by adding high atomic weight elements, such as rhenium (Re), as an integral component of such coatings. Overlay coatings comprised of MCrAlY laced with rhenium are reported to have increased oxidation resistance and decreased thermal and material degradation.
Although some success has been reported when Re is used as an integral additive in MCrAlY overlay coatings, the use of Re as an integral additive to the coating necessarily results in random distribution of Re atoms throughout the overlay coating. Some of the constituent aluminum atoms in the coating necessarily will diffuse past such randomly dispersed Re atoms and out of the overlay coating.
Some have attempted to form rhenium diffusion barriers by simply applying a coating of rhenium to the surface of a superalloy substrate. A simple rhenium coating is not effective to prevent diffusion between a superalloy substrate and an overlay coating while maintaining structural integrity under the rigorous conditions that would be encountered by "high intensity" superalloy components. Simple rhenium coatings are brittle, continuous, and unstable, and have very often been attempted and found not to work.
The use of electron beam physical vapor deposition to deposit rhenium onto the superalloy substrate, coupled with the use of an ion beam as a source of energy to cause rhenium atoms to actually penetrate and diffuse into the substrate, also has been suggested. The result is said to be a coating of rhenium atoms at the surface of the superalloy substrate and an adjacent interfacial layer comprising a mixture of superalloy atoms and rhenium atoms which adhere the coating to the substrate and are uniquely positioned in the substrate to slow diffusion between the substrate and the overlay coating.
For components with complex geometries, it can be difficult to achieve a uniform diffusion barrier using ion beam assisted deposition. Alternative methods are needed for depositing diffusion barriers which are capable of achieving more uniformity.